1. Field of the Invention
The present invention relates to an image reading apparatus that reads an image on a document, and more particularly to an image reading apparatus that includes a reference member for conducting shading correction.
2. Description of the Related Art
It is known to provide image reading apparatuses that automatically read image information of a document are used as reading devices, such as those of copiers and fax machines, and as scanners for computer input. This type of image reading apparatus reads an image on a document by irradiating the document with light, using a light source that extends in a direction orthogonal to the document conveyance path, and receiving reflection light reflected from the irradiated document with an image sensor.
With this kind of image reading apparatus, situations arise where there are variations in the amount of light emitted from the light source, due to its position in the direction orthogonal to the document conveyance path, and the amount of light of the light source fluctuates over time. When fluctuations in the amount of light arise, such as when the image reading apparatus reads a document on which a half-tone image is uniformly formed, the outputted image signal becomes nonuniform and includes variations corresponding to the amount of light of the light source.
Thus, a technique is widely used where a white member (white reference member) that extends in the direction orthogonal to the document conveyance path is disposed facing the light source, the white reference member is irradiated with light from the light source, an image sensor receives reflection light reflected from the white reference member, and correction data (shading data) corresponding to the illumination distribution in the direction orthogonal to the document conveyance direction are acquired in advance. Then, at the time the document is actually read, the image data obtained by reading the document are corrected (shading correction) using the shading data, whereby irregularities resulting from the light amount distribution of the light source are removed.
There is the potential for dust and dirt to adhere to the white reference member, and when shading correction of the read image is conducted with the shading data obtained by reading the white reference member in this state, stripes arise at sites corresponding to the portions to which the dust and dirt are adherent. Thus, conventionally, image reading apparatuses have been configured so that the shading data is acquired at plural positions of the white reference member by moving the sensor or a mirror, and the whitest data are used as the shading data.
However, in the known image reading apparatus, it is difficult to acquire the shading data at plural positions of the white reference member because the image sensor and the white reference member are fixedly disposed.